1. Field of the Invention
The present invention relates to a material for an insulating film, a coating varnish for an insulating film and an insulating film and a semiconductor using the material or the varnish. More particularly, the present invention relates to a material for an insulating film which exhibits excellent electrical, thermal and mechanical properties, can achieve a low permittivity and is advantageously used for insulating interlayer films and surface protective films in semiconductors, insulating interlayer films of multi-layer circuits, cover coats of flexible copper clad laminates, solder resist films and liquid crystal-aligning films, a coating varnish for an insulating film comprising this material, an insulating film using the material or the varnish and a semiconductor using the material or the varnish.
2. Description of Related Art
As the materials for semiconductors, inorganic materials and organic materials are used in various portions in accordance with the necessary properties. For example, as the insulating interlayer film for semiconductors, films of inorganic oxides such as silicon dioxide prepared in accordance with the chemical vapor deposition process are used. However, as the speed of semiconductors increases and the performance is still more improved, a problem arises with the films of inorganic oxides in that the permittivity is high. As a method to overcome this problem, application of organic materials has been studied.
The organic materials applied to semiconductors are, for example, polyimide resins exhibiting excellent heat resistance and electrical and mechanical properties. The polyimide resins are used for solder resist films, cover lays and liquid crystal-aligning films. However, since the polyimide resins have two carbonyl groups on the imide ring, in general, problems are found with respect to water absorption and electrical properties. To overcome these problems, improvement in the water absorption and the electrical properties has been attempted by introducing fluorine or groups having fluorine into the organic macromolecule and some of the improved resins are practically used. Polybenzoxazole resins are the resins exhibiting more excellent heat resistance, water absorption and electrical properties than those of the polyimide resins. Application of the polybenzoxazole resins to various fields has also been attempted. Examples of the polybenzoxazole resin include resins having the structure derived from 4,4′-diamino-3,3′-dihydroxybiphenyl and terephthalic acid and resins having the structure derived from 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane and terephthalic acid.
However, it is the actual present situation that, in the most advanced field requiring more excellent properties such as more excellent heat resistance, electrical properties and water absorption, no materials satisfying the all requirements have been obtained yet. Some materials exhibit insufficient electrical properties such as permittivity although the heat resistance is excellent. The introduction of fluorine causes a decrease in the heat resistance although the electrical properties can be improved. In particular, when organic materials are applied to insulating interlayer films for semiconductors, it is required that the heat resistance, the mechanical properties and the water absorption be as excellent as those of inorganic materials and, moreover, a lower permittivity is required.
In response to the requirements for further improvements in the properties, decreasing density of films of inorganic oxides, which are inorganic materials, by forming fine pores in the films is studied in order to achieve a decrease in the specific permittivity. The air has a specific permittivity of 1. Therefore, the decrease in the specific permittivity by introduction of the air in the film may be suggested by the method described in the specification of the U.S. Pat. No. 3,883,452 in which a foamed polymer having pores having an average diameter of about 20 μm is formed. However, to prepare an effective insulating material by introducing the air into a film, it is necessary that a uniform distribution of the specific permittivity be achieved in. a film having a thickness of an order of submicrometer. It is also necessary that the film have sufficient mechanical properties for enduring various steps in the production process. It is the actual present situation that no inorganic materials overcoming the above problems have been obtained yet.
On the other hand, as the technology for obtaining fine pores having a size of an order of submicrometer in organic materials, a technology in which a resin having fine pores having a size of an order of submicrometer is formed by a heat treatment of a block copolymer is disclosed (U.S. Pat. No. 5,776,990). The phase separation of a size of an order of submicrometer in block copolymers has been known [T. Hashimoto, M. Shibayama, M. Fujimura and H. Kawai, “Microphase Separation and the Polymer-polymer Interphase in Block Polymers” in “Block Copolymers-Science and Technology”, p.63, Ed. by D. J. Meier (Academic Pub., 1983)]. It is also well known in the field of the macromolecular chemistry that polymers having a low ceiling temperature are easily decomposed. However, to obtain a resin composition having fine pores while the requirements not only for the specific permittivity but also for the mechanical properties, the electrical properties, the water absorption and the heat resistance are satisfied, the combination of the resin, the technology of forming blocks and the components decomposable under heating is extremely limited. It is the actual present situation that no technology satisfying the entire requirements can be found.